Organic photoconductors



United States Patent Olhce U.S. Cl. 961.6 Claims ABSTRACT OF THE DISCLOSURE 3,3 bis 1,5-diaryl pyrazolines are useful as organic photoconductors in electrophotographic elements.

This invention relates to electrophotography, and in particular to photoconductive compositions and elements.

The process of xerography, as disclosed by Carlson in US. 2,297,691, employs an electrophotographic element comprising a support material bearing a coating of a normally insulating material whose electrical resistance varies with the amount of incident actinic radiation it receives during an imagewise exposure. The element, commonly termed a photoconductive element, is first given a uniform surface charge, generally in the dark after a suitable period of dark adaptation. It is then exposed to a pattern of actinic radiation which has the effect of differentially reducing the potential of the surface charge in accordance with the relative energy contained in various parts of the radiation pattern. The differential surface charge or electrostatic latent image remaining on the electrophotographic element is then made visible by contacting the surface with a suitable electroscopic marking material. Such marking material or toner, whether contained in an insulating liquid or on a dry carrier, can be deposited on the exposed surface in accordance with either the charge pattern or the discharge pattern as desired. The deposited marking material can then be either permanently fixed to the surface of the sensitive element by known means such as heat, pressure, solvent vapor, or the like, or transferred to a second element to which it can similarly be fixed. Likewise, the electrostatic latent image can be transferred to a second element and developed there.

Various photoconductive insulating materials have been employed in the manufacture of electrophotographic elements. For example, vapors of selenium and vapors of selenium alloys deposited on a suitable support and particles of photoconductive zinc oxide held in a resinous, film-forming binder have found Wide application in present-day document copying applications.

Since the introduction of electrophotography, a great many organic compounds have also been screened for their photoconductive properties. As a result, a very large number of organic compounds are known to possess some degree of photoconductivity. Many organic compounds have revealed a useful level of photoconduction and have been incorporated into photoconductive compositions. Optically clear organic photoconductor-containing elements having desirable electrophotographic properties can be especially useful in electrophotography. Such electrophotographic elements can be exposed through a transparent base if desired, thereby providing unusual flexibility in equipment design. Such compositions, when coated as a film or layer on a suitable support also yield an element which is reusable; that is, it can be used to form subsequent images after residual toner from prior images has been removed by transfer and/or cleaning. Thus far, the selection of organic compounds for incorporation into photoconductive compositions to form electrophotographic layers has proceeded on a compound-by-com- 3,527,602 Patented Sept. 8, 1970 pound basis. Nothing has yet been discovered from the large number of different photoconductive substances tested which permits effective prediction and therefore selection of particular compounds exhibiting the desired electrophotographic properties.

It is, therefore, an object of this invention to provide photoconductive elements for use in electrophotography containing a novel class of organic photoconductors having enhanced photosensitivity when electrically charged.

It is also an object to provide electrophotographic elements having a layer of a novel photoconductive composition which can be positively or negatively charged.

It is another object to provide novel transparent electrophotographic elements having high speed characteristics.

It is a further object of this invention to provide novel electrophotographic elements useful for producing images electrophotographically by reflex or birefiex processes.

These and other objects of this invention are accomplished with electrophotographic elements having coated thereon organic photoconductive compositions containing an organic compound having a 3,3'-bis-aryl-2- pyrazoline nucleus.

The 3,3'-bis-aryl-2-pyrazoline nucleus can be substituted in either five-member ring with the same or different substituents. The 1 and 5 positions on both pyrazoline rings are substituted by an aryl moiety including unsubstituted as well as substituted aryl substituents such as alkoxyaryl, alkaryl, alkaminoaryl, carboxyaryl, hydroxyaryl and haloaryl. The 4 position can contain hydrogen or unsubstituted as Well as substituted alkyl and aryl radicals such as alkoxyaryl, alkaryl, alkaminoaryl, haloaryl, hydroxyaryl, alkoxyalkyl, aminoalkyl, carboxyaryl, hydroxyalkyl and haloalkyl.

The preferred photoconductors of this invention are represented by the following structure:

wherein:

D, D, J and I can be either a phenyl radical including a substituted phenyl radical such as a tolyl radical or a naphthyl radical including a substituted naphthyl radical,

E, E, G, G, L and L' can be any of the substituents set forth above and in addition can be either a hydrogen atom or an alkyl radical containing 1-8 carbon atoms.

Some typical photoconductors of this invention are:

( 1) 3,3-bis(1,S-diphenyl-Z-pyrazoline);

(2) 3,3-bis( 1-p-tolyl-5-phenyl-2-pyrazoline);

(3) 3,3-bis(1,5-[l-naphthyl1-2-pyrazoline);

(4) 1,5 -diphenyl-3- [3'-( l-p-tolyl-5 '-phenyl) -2'-pyrazolyl] -2pyrazoline;

( 5) 3 ,3-bis( 1,5 -diphenyl-4,5 -dimethyl-2-pyrazoline) (6) 3,3 '-bis( 1,4,5 -triphenyl-2-pyrazoline) (7 3,3-bis( l ,5-di-p-tolyl-4-methoxy-2-pyrazoline) (8) 3 ,3 bis 1,5 -di'phenyl-4-dimethylamino-2-pyrazoline) (9) 3,3'-bis[ 1,5-diphenyl-4-(p-chlorophenyl)-2-pyrazoline];

( 10) 3,3 '-bis 1,5 -diphenyl-4,5 -dip-diethylaminophenyl) -2-pyrazoline] (11) 3,3 -bis[1,5-diphenyl-4-(p-methoxyphenyl)-5- ethyl-2-pyrazoline] 12) 3,3-bis(1,5-diphenyl-4-chloromethyl-2-pyrazoline); and

(13) 1,5-diphenyl-4,5-dimethyl-3-[3'-1-p-tolyl-4'-diethyl-5,5'-methylphenyl)-2-pyrazolyl]-2-pyraz0line.

These compounds are prepared by the method set forth in copending application Ser. No. 664,629, filed concurrently herewith on Aug. 31, 1967, by James K. Elwood, now abandoned.

Electrophotographic elements of the invention can be prepared with these photoconducting compounds in the usual manner, i.e., by blending a dispersion or solution of a photoconductive compound together with a binder, when necessary or desirable, and coating or forming a self-supporting layer with the photoconductor-containing materials. Mixtures of the photoconductors described herein can be employed. Likewise, other photoconductors known in the art can be combined with the present photoconductors. In addition, supplemental materials useful for changing the spectral sensitivity or electrophotosensitivity of the elements can be added to the composition of the element when it is desirable to produce the characteristic effect of such materials.

Sensitizing compounds useful with the photoconductive compounds of the present invention can include a wide variety of substances such as pyrylium, thiapyrylium, and selenapyrylium salts of U.S. Pat. 3,250,615, issued May 10, 1966; fluorenes, such as 7,12-diox0-13-dibenzo(a,h) fiuorene, 5,10-dioxo-4a,11-diazabenzo(b)fiuorene, 3,13- dioxo 7 oxadibenzo(b,g)-fluorene, trinitrofluorenone, tetranitrofiuorenone and the like; aromatic nitro compounds of U.S. Pat. 2,610,120; anthrones of U.S. Pat. 2,670,285; quinones of U.S. Pat. 2,670,286; benzophenones of U.S. Pat. 2,670,287; thiazoles of U.S. Pat. 2,732,301; mineral acids; carboxylic acids, such a maleic acid, dichloroacetic acid, and salicylic acid; sulfonic and phosphoric acids; and various dyes such as triphenylmethane, diarylmethane, thiazine, azine, oxazine, Xanthene, phthalein, acridine, azo, anthraquinone dyes and many other suitable sensitizing dyes. The preferred sensitizers for use with the compounds of this invention are pyrylium and thiapyrylium salts, fluorenes, carboxylic acids, and triphenylmethane dyes.

Where a sensitizing compound is to be used within a photoconductive layer as disclosed herein it is conventional practice to mix a suitable amount of the sensitizing compounds with the coating composition so that, after thorough mixing, the sensitizing compound is uniformly distributed throughout the desired layer of the coated element. In preparing the photoconducting layers, no sensitizing compound is needed for the layer to exhibit photoconductivity. The lower limit of sensitizer required in a particular photoconductive layer is, therefore, zero. However, since relatively minor amounts of sensitizing compound give substantial improvement in the electrophotographic speed of such layers, the use of some sensitizer is preferred. The amount of sensitizer that can be added to a photoconductor-incorporating layer to give effective increases in speed can vary widely. The optimum concentration in any given case will vary with the specific photoconductor and sensitizing compound used. In general, substantial speed gains can be obtained where an appropriate sensitizer is added in a concentration range from about 0.0001 to about 30 percent by weight based on the weight of the film-forming coating composition. Normally, a sensitizer is added to the coating composition in an amount by weight from about 0.005 to about 5.0 percent by weight of the total coating composition.

Preferred binders for use in preparing the present photoconductive layers are film-forming polymeric binders having fairly high dielectric strength which are good electrically insulating film-forming vehicles. Materials of this type comprise styrene-butadiene copolymers; silicone resins; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins; poly(vinyl chloride); poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; poly(vinyl acetate); vinyl acetate-vinyl chloride copolymers; poly (vinyl acetals), such as poly(vinyl butyral); polyacrylic and methacrylic esters, such as poly(methylmethacrylate), poly (n-butylmethacrylate) poly (isobutyl methacrylate) etc.; polystyrene; nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such as poly (ethylenealkaryloxyalkylene terephthalate); phenolformaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly(ethyleneglycol-co-bishydroxyethoxyphenyl propane terephthalate); etc. Methods of making resins of this type have been described in the prior art, for example, styrene-alkyd resins can be prepared according to the method described in U.S. Pats. 2,361,019 and 2,258,423. Suitable resins of the type contemplated for use in the photoconductive layers of the invention are sold under such trade names as Vitel PE-lOl, Cymac, Piccopale 100, Saran F-220 and Lexan 105. Other types of binders which can be used in the photoconductive layers of the invention include such materials as parafiin, mineral waxes, etc.

Solvents of choice for preparing coating compositions of the present invention can include a number of solvents such as benzene, toluene, acetone, 2-butanone, chlorinated hydrocarbons, e.g., methylene chloride, ethylene chlo ride, etc., ethers, e.g., tetrahydrofuran, or mixtures of these solvents etc.

In preparing the coating composition useful results are obtained where the photoconductor substance is present in an amount equal to at least about 1 weight percent of the coating composition. The upper limit in the amount of photoconductor substance present can be widely varied in accordance with usual practice. In those cases where a binder is employed, it is normally required that the photoconductor substance be present in an amount from about 1 weight percent of the coating composition to about 99 weight percent of the coating composition. A preferred weight range for the photoconductor substance in the coating composition is from about 10 Weight percent to about 60 weight percent.

Coating thicknesses of the photoconductive composition on a support can vary widely. Normally, a coating in the range of about 0.001 inch to about 0.01 inch before drying is useful for the practice of this invention. The preferred range of coating thickness was found to be in the range from about 0.002 inch to about 0.006 inch before drying although useful results can be obtained outside of this range.

Suitable supporting materials for coating the photoconductive layers of the present invention can include any of a wide variety of electrically conducting supports, for example, paper (at a relative humidity above 20 percent); aluminum-paper laminates; metal foils such as aluminum foil, zinc foil, etc.; metal plates, such as aluminum, copper, zinc, brass, and galvanized plates; vapor deposited metal layers such as silver, nickel, or aluminum and the like. An especially useful conducting support can be prepared by coating a support material such as polyethylene terephthalate with a layer containing a semiconductor dispersed in a resin. Such conducting layers both with and without insulating barrier layers are described in U.S. Pat. 3,245,833. Likewise, a suitable conducting coating can be prepared from the sodium salt of a carboxyester lactone of maleic anhydride and a vinyl acetate polymer. Such kinds of conducting layers and methods for their optimum preparation and use are disclosed in U.S. 3,007,901 and 3,267,807.

The elements of the present invention can be employed in any of the well-known electrophotographic processes which require photoconductive layers. One such process is the aforementioned Xerographic process. As previously explained, in a process of this type the electrophotographic element is given a blanket electrostatic charge by placing the same under a corona discharge which serves to give a uniform charge to the surface of the photoconductive layer. This charge is retained by the layer owing to the substantial insulating property of the layer, i.e., the low conductivtiy of the layer in the dark. The electrostatic charge formed on the surface of the photoconducting layer is then selectively dissipated from the surface of the layer by exposure to light through an 6 EXAMPLE 1 Organic photoconductors of the type described herein are separately incorporated into a coating dope having the following composition:

. 5 lmage-bearing transparency by a conventional exposure operation such as, for example, by contact-printing techgg fig g g% nique, or by lens projection of an image, etc., to form a ia z 8 latent image in the photoconducting layer. By exposure Meth m1 of the surface in this manner, a charged pattern is creat- 10 y ed by vlrtue of the.faet that .hght cause? the .eharge to The resulting compositions are handcoated at a wet thickbe conducted away In proport o 0 the mtenslty of the ness of 0.004 inch on a conducting layer comprising the illummatlon m a partlelilar area The charge Pattern sodium salt of a carboxyester lactone, such as described f after exposure then developed rendered in U.S. 3,120,028, which in turn is coated on a cellulose l by treatment Wlth a medlum colinpnsing electro acetate film base. The coating blocks are maintained at a statlcally attractable particles having optical density. The temperature of 900 R These elcctrophotographic elements developing electrostatically attractable particles can be are charged under a positive or negative corona source m F of a dust i plgment m a resmous until the surface potentials, as measured by an electromtoner a hqmd developer. y be used eter probe reach between about 500 and 600 volts. They in WhlCh the developing particles are carried in an elec- 20 are then gubjectfid to exposure from behind a Stepped trically insulating liquid carrier. Methods of develop density gray scale to a 0 tungsten source The ment ofthrs type are wldely knpWn and have been exposure causes reduction of the surface potentials of Senbed the patent hterature m e patentsafor the elements under each step of the gray scale from their ample as 2297691 and In Auttrahan initial potential, V to some lower potential, V whose ex- In processes of electmphclmgraphlc reprodue' act value depends on the actual amount of exposure in Sud} as in xeregraphyflby eeleetmg a develop mg F meter-candle-seconds received by the areas. The results i 9 has one of Its components a low'meltmg of the measurements are plotted on a graph of surface po- 9 It possible to treat the developed photoeondue' tential V vs. log exposure for each step. The speed is the eve matenal wlth heat and cause the powder adhere numerical expression of 10 multiplied by the reciprocal Permanently to the surface of the Pmmwndmwe layer of the exposure in meter-candle-seconds required to reduce In other e a transfer of the Image formed on the the 500 to 600 volt charged surface potentials by 100 volts. ph9tcnductwe layer can be made R second .support This speed is related to the fringe development of the Whleh i then become the final P Teehmques of latent image, higher speeds being an indication of good the type i are known m the and have fringe development and lower speeds indicating little or been described 1n a number of US. and fore1gn patents, 35 no h5g6 development The Speeds of the Various Photo 3 22;: P "2' k ggg gg g 3 22 and m RCA conductive compositions are shown in Table I below. The i I e e The present invention is not limited to any particular sens1t1zers referred to in the table are as follows. mode of use of the new electrophotographic materials, sensitizer added and the exposure technique, the charging method, the 40 transfer (if any), the developing method, and the fixing B21Ze'b1e(4eiti1y1pheny1)'4e(4'emylexyphenyl)thlepy method as well as the materials used in these methods can Ty mm i Grate be selected and adapted to the requirements of any particular technique. D C1'Ysta1 Ylolet Electrophotographic materials according to the pres- E Rhdamme B ent invention can be applied to re reduction techniques wherein different kinds of radiations, i.e., electromagnetic The bmder a polyester of terephththc aeld and e i radiations as well as nuclear radiations, can be used. For ture of ethylene glycol (1 part by Welght) this reason, it is pointed out herein that although matehydroxyePhoxyphenyl)Propane (9 Partf by Welght) sold rials according to the invention are mainly intended for commel'clany under h trade name Vltel Where the use in connection with methods comprising an exposure, Y -PY are employed as P the term electrophotography wherever appearing in the tors in the following table, the low melting fraction 18 used description and the claims, is to be interpreted broadly since it generally has better photoconducting properties and understood to comprise both xerography and xerothan the higher melting fraction. The fractions are obradiography. tained in accordance with the procedure set forth in the The invention is further illustrated by the following expreviously mentioned co-pending application Ser. No. amples which include preferred embodiments thereof. 664,629.

TABLE I H2C---C-CCH2 J, l l (L l \E/ IL I Substituents Speed Com- Sensi- Ex. pound D D .T J tizer Pos Neg 1a I Phenyl Pheny1 Phenyl Phenyl lg 9, ggg 5153 lb II p-T0lyl..... p-Tolyl d0 .-do g g 0 630 D 630 400 E 500 320 1c III Phenyl Phenyl...... H H g 28 3 TABLE II HzC-CR K H t D Substituents Speed Com- Ex. pound R D J Sensitizer Pos. Neg

1d IV Phenyl Phenyl Phenyl A 160 C 380 1e V H do do A 160 0 B 80 40 O 160 5.0 If VI -CHO d0 .-do A 4 0 G 200 5.0

Cells 1g VII CH=CH-C .do ..d0 A 0 0 II B 32 32 O C 16 1h VIII --CH=NNH -do do- A 6.3 0 B 32 25 Examples 1a and lb of Table I illustrate the electrical speeds of the elements containing the photoconductive compounds of this invention. An analysis of the electrical speeds of the remaining elements (c-h) set forth in Tables I and II indicates that in order for the composition to exhibit photoconductivity to an appreciable extent, the compound must (1) be substituted in the 1 and 5 positions by an aryl substituent and (2) be substituted in the 3 position by a corresponding 2-pyrazoly1 moiety so that the resultant structure contains a 3,3'-bis-aryl-2-pyrazoline nucleus. Compounds with other substituents in the 3 position such as set forth in Table II by compounds IV- VIII do not impart the substantial photoconductivity to the composition when compared to that imparted by the compounds of this invention as indicated by the electrical speeds, even though the 1 and 5 positions contain aryl substituents.

EXAMPLE 2 Coating dopes prepared in the manner set forth in Example 1 containing compounds I and II are coated in the manner described in Example 1. In a darkened room, the surface of each of the photoconductive layers so prepared is charged to a potential of about +600 volts under a corona charger. The layer is then covered with a trans parent sheet bearing a pattern of opaque and light transmitting areas and exposed to the radiation from an incandescent lamp with an illumination intensity of about 75 meter-candles for 12 seconds. The resulting electrostatic latent image is developed in the usual manner by cascading over the surface of the layer a mixture of negatively charged black thermoplastic toner particles and glass beads. A good reproduction of the pattern results in each instance.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be efiected within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.

We claim:

1. An electrophotographic element comprising a support having coated thereon a photoconductive composition comprising a photoconductor which is a 3,3-bis-(l,5- diaryl-Z-pyrazoline) 2. An electrophotographic element as defined by claim 1 wherein the photoconductive composition is sensitized with a sensitizer selected from the group consisting of a pyrylium salt, a thiapyrylium salt, a selenapyrylium salt, a fluorene, a carboxylic acid and a triphenyl methane dye.

3. An electrophotographic element comprising a support having coated thereon a photoconductive composition comprising a photoconductor having the structure:

wherein 1) D, D, I, and J are each an aryl radical and (2) E, E, G, G, L and L' are each selected from the group consisting of a hydrogen atom, an alkyl radical and an aryl radical. 4. An electrophotographic element as defined by claim 3 wherein the photoconductive composition is sensitized with a sensitizer selected from the group consisting of a pyrylium salt, a thiapyrylium salt, a selenapyrylium salt, a fluorene, a carboxylic acid and a triphenyl methane dye.

5. An electrophotographic element comprising a support having coated thereon a photoconductive composition comprising a photoconductor having the structure:

D and D are each selected from the group consisting of a phenyl radical and a tolyl radical and I and J' are each a phenyl radical.

6. An electrophotographic element as defined by claim 5 wherein the photoconductive composition is sensitized with a sensitizer selected from the group consisting of a pyrylium salt, a thiapyrylium salt, a selenapyrylium salt, a fluorene, a carboxylic acid and a triphenyl methane dye.

7. An electrophotographic element comprising a support having coated thereon a photoconductive composition comprising a polymeric binder and a photoconductor selected from the group consisting of 3,3-bis-(l,5-diphenyl-Z-pyrazoline) and 3,3-bis-l-p-tolyl 5-phenyl 2- pyrazoline) 8. A photoconductive element for use in electrophotography ocmprising a support having coated thereon a photoconductive composition comprising:

(a) about 10 to 60% by weight based on said photoconductive composition of 3,3'-bis-(1,5-diphcnyl-2- pyrazoline) as a photoconductor,

(b) a film-forming polymeric binder for the said photoconductor, and

(c) a sensitizer for the said photoconductor.

9. A photoconductive element for use in electrophotography comprising a support having coated thereon a photoconductive composition comprising:

(a) about 10 to 60% by weight based on said photoconductive composition of 3,3-bis-(l-p-tolyl-S-pheny1-2-pyrazoline),

(b) a film-forming polymeric binder for the said photoconductor, and

(c) a sensitizer for the said photoconductor.

10. In an electrophotographic process wherein an electrostatic charge pattern is formed on a photoconductive element, the improvement characterized in that said photoconductive element has a photoconductive layer comprising an organic compound having a 3,3'-bis-(1,5- diary1-2-pyrazoline) nucleus.

References Cited UNITED STATES PATENTS 3,180,729 4/1965 Klupfel et al. 961.5 3,250,615 5/1966 Van Allen et a1. 961.7 3,287,122 11/1966 Hoegl 961.5 3,331,687 7/1967 Kosche 961.5

FOREIGN PATENTS 630,038 12/1963 Belgium.

GEORGE F. LESMES, Primary Examiner C. E. VAN HORN, Assistant Examiner U.S. Cl. X.R. 961, 1.5; 260-310 

